1. Field of the Invention
The present invention relates to a method of and an apparatus for curing an optical disc (hereinafter referred to as an optical disc curing method and an optical disc curing apparatus), particularly to a method of and apparatus for curing an optical disc comprising first and second resin substrates which are overlaid with each other by way of an adhesive.
2. Prior Art
Computers have been now used in various fields, and they have a leap forward in development of processing capacity and speed. Particularly, storage density of memories to be used has improved dramatically, impacting daily life to change a common sense in the daily life. Among memories, a so-called storage disc, particularly, an optical disc is convenient to be carried and has various uses as storage means.
The optical disc is preferably formed of a discoid substrate made of synthetic resin such as polycarbonate.
Signal pits that are information signals, hereinafter referred to as information, are stored in the resin substrate, and a reflecting film made of aluminum or the like is formed on the resin substrate. A protecting resin layer made of, e.g., ultraviolet curing resin (so-called ultraviolet resin) is formed on the reflecting film for protecting the resin substrate in which information is stored.
Whereupon, a thickness of the resin substrate is set to 0.6 mm in view of the recording of the signal pits, the manufacturing limits, and strength of the resin substrate.
Since a CD has a thickness of about 1.2 mm as its standard, it is very useful for two resin substrates to be bonded to each other by an adhesive to conform to the thickness of the CD.
An optical disc D shown in FIG. 5 typically comprises a first resin substrate U1 composed of a resin substrate in which information is having a reflection film a and a protecting resin layer b formed on the reflecting film a, and a second dummy resin substrate U2 in which no information is stored, wherein the second dummy resin substrate U2 is bonded to the first resin substrate U1.
An optical disc D shown in FIG. 6 comprises two resin substrates in which different information is stored are bonded to each other.
The optical disc D in FIG. 6 comprises first and second resin substrates U1 and U2 each composed of a resin substrate in which information is stored having a reflection film a formed on the resin substrate and a protecting resin formed on the reflecting film a, wherein the first and second resin substrates U1 and U2 are bonded to each other by way of an adhesive R.
The optical disc having both resin substrates in which information is inputted is largely expected hereinafter as a digital video disc (DVD) sometimes referred to as a digital versatile disc. Whereupon, the inventors of the present application manufacture the optical disc in the following manner.
FIGS. 7 (1) through (5) show schematic views for explaining the processes to manufacture the optical disc D by bonding two resin substrates. These procedures are now briefly described in the order of the procedures.
First, a first resin substrate U1 in which information is stored is placed on a receiving table S (Process 1).
The receiving table S has a boss at the center thereof for positioning the resin substrate U1 and is rotatable at high speed (a boss is omitted in FIG. 7). Successively, the first resin substrate U1 is placed on the receiving table S, and an adhesive R such as ultraviolet curing resin, etc. on the first resin substrate U1 (Process 2).
When the ultraviolet curing resin is used as the adhesive R, coating is placed onto the first resin substrate U1 by way of a nozzle so as to pour the adhesive R therefrom onto the first resin substrate U1. For example, if a jet nozzle N is moved in a state where the first resin substrate U1 is turned, the adhesive R can be coated onto the first resin substrate U1 in a doughnut shape. After the adhesive R is coated onto the first resin substrate U1, the second resin substrate U2 can be overlaid on the first resin substrate U1 (Process 3).
Thereafter, the overlaid two resin substrates are integrally turned at high speed (Process 4).
That is, the adhesive interposed between the first and second resin substrates U1 and U2 is developed to extend uniformly between the first and second resin substrates U1 and U2.
Air sealed between the first and second resin substrates are blown off in a state where both resin substrates are overlaid onto each other during rotation. Further, an air bubble which is contained in the adhesive per se, if any, also escapes outside the adhesive.
In the developing process, a large quantity of the adhesive R interposed between both resin substrates is scattered from the circumferential peripheries of both resin substrates when the receiving table S is turned at high speed. Although the developing time is several seconds, the adhesive is strongly scattered at an acceleration time, before the rotating speed is changed to a high rotating speed, namely, within one to two seconds of several seconds of the developing time.
After the adhesive R is developed, it is cured in the next process. When the ultraviolet curing resin is used as the adhesive R, the resin is irradiated with ultraviolet radiation (hereafter ultraviolet) which is irradiated from the upper portion of the second resin substrate. When the ultraviolet curing resin is cured upon reception of the ultraviolet so that the first and second resin substrates U1 and U2 are integrated with each other, thereby forming a strongly bonded optical disc.
The inventors of the present application have manufactured the optical disc by bonding the first and second resin substrates in the manner as set forth above.
Meanwhile, when the adhesive is cured, the optical disc is placed on the receiving table and it is irradiated with the ultraviolet from the upper portion of the optical disc, it is understood that the optical disc is subject to a warp phenomenon, namely, it is liable to be warped.
That is, if the optical disc comprises both resin substrates in which information is respectively stored or even if it comprises both resin substrates in any of which information is stored, when the adhesive is cured when it is irradiated with the ultraviolet from, e.g., the upper portion of the resin substrates, a warp phenomenon is generated, namely, the optical disc per se is bent upward.
The optical disc which was subject to the warp phenomenon, namely, the warped optical disc is deteriorated in quality, and hence the warp phenomenon must be avoided. On the other hand, when the optical disc is manufactured, the curing time must be as short as possible to enhance the productivity. Accordingly, the intensity of the ultraviolet is increased so as to cure the adhesive. However, when the intensity of the ultraviolet is increased, we found that there was a tendency that the optical disc was largely warped. As a result, in the conventional curing method, there is a limit to expedite the curing speed, leading to deterioration of productivity.